General Terms

COLOR  TEMPERATURE

Josef Stefan 
An Austrian physicist
b-1835-03-24
St. Peter in Austria  d-1893-01-07
Vienna in Austria

In 1879 Josef Stefan formulated a law which states that the radiant energy of a blackbody 
(a theoretical object that absorbs all radiation that falls on it) is proportional to the 
fourth power of its temperature. His law was one of the first important steps 
toward the understanding of blackbody radiation.

A BLACKBODY is a surface that absorbs all radiant energy falling on it. The term arises because incident visible light will be absorbed rather than reflected, and therefore the surface will appear black. The concept of such a perfect absorber of energy is extremely useful in the study of radiation phenomena. The best practical blackbody is a small hole in a box with a blackened interior, because practically none of the radiation entering such a hole could escape again, and it would be absorbed inside. A surface covered with lampblack will absorb about 97 percent of the incident light and can be considered a blackbody.

Lord Rayleigh 
A British physical scientist
b-1842-11-12
Langford Grove, Maldon, Essex in England 
d-1919-06-30
Terling Place, Witham, Essex in England

Sir James Jeans
An English
physicist and mathematician
b-1877-09-11
London, England 
d-1946-09-16
Dorking, Surrey, England

Lord Rayleigh first attempted to describe this distribution. 
He modeled the blackbody by assuming that it consisted of a collection of oscillators that 
could absorb and emit electromagnetic radiation at any frequency.

 
By statistical processes he arrived at a certain equation, which when he presented it publicly, an audience member by the name of Sir James Jeans stood and said "It seems to me that Lord Rayleigh had introduced an unnecessary factor 8 by counting negative as well as positive integers."

 
With that small correction came the celebrated (but still very wrong) Rayleigh-Jeans Law. 
The main problem with this is that though it fit the data very well at low energies (the distribution of light in the infrared region), it predicted a growth without bound as one went towards the ultraviolet end of the spectrum. 
This was called the "Ultraviolet Catastrophe" since it predicted that the world should be filled with X-rays and Gamma Rays and we all should have been fried eons ago.

When we look at the heating element in an oven, hot objects give off light. In fact everything at any temperature emits light. We don't usually associate this with everyday objects because the light coming from objects at room temperature is far into the infrared where we can't see it. An object which gives off a certain distribution of light at the lowest possible temperature is called a BLACKBODY. At a given temperature, 
all objects give off a "redder" light distribution than the blackbody at the same temperature, and so appear to be "cooler". A large, heated cavity with a tiny hole is often presented as being excellent approximation to such an ideal blackbody. The spectral distribution is measured by allowing the light inside to escape out the box through the tiny hole. The hole needs to be small compared to the size of the box so that the amount of energy escaping is negligible compared to all the energy inside the box. Then the measurement doesn't disturb the distribution.

Planck Max, Karl, Ernst 
A German physicist Nobel prize 1918
b-1858-04-23
Kiel, Germany 
d-1947-10-03
Göttingen, Germany

Max Planck played around with the equations and came across a mathematical form that exactly predicted the observed blackbody spectra. However, as he tried to understand what it meant, his only conclusion was that the oscillators making up the system could not absorb radiation of any frequency. Instead they had to absorb and emit it in only discrete quantities - that the energy of the oscillators had to be quantized. 

This was the birth of the quantum theory. No explanation of WHY they were quantized was given but it certainly predicted exactly the observed blackbody emission spectra. Others undertook to explore this concept, though Planck himself never believed it was physically real. He only thought that he had developed a mathematical trick that worked. But he still gave the birth to the quantum theory.

How to characterize the difference between light sources? 
The standard approach is to compare a light source to a blackbody source and characterize the light source by the temperature of the blackbody radiator and plot the result as x and y on the CIE-1931 Chromaticity Diagram.

In practice, a number of standard illuminants are used.
For accurate color measurement, one needs more than 
only the color temperature.
The color temperature is a useful concept to reveal the general color balance of the source.
Note that a high color temperature does not imply that the object is actually at that temperature.
For example, fluorescent light have a high color temperature, but actually 
may be quite cool in operation. 

A blackbody radiator above 2500 K is 
in the general "white" area of the Chromaticity Diagram.
Light sources are usually characterized by their color temperature.

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 Last update
2008-08-20